1. Field of the Invention
This invention relates to diagnosing internal combustion engines electronically.
2. Description of the Prior Art
In diesel engines, and other engines which do not utilize a spark ignition, it is difficult to provide adequate speed indications. In one case known in the art, a signal which appears once in each cycle of the engine (one revolution in a two stroke engine or two revolutions in a four stroke engine) is utilized to control counting of time increments from one cycle to the next, so that the average speed of each preceding cycle can be measured. This manner of measuring speed is adequate only when average speed over a cycle is desired, and only when the purpose of the speed measurement occurs only on the boundaries of the cycles across which the average speed is measured and is not concerned with variations in speed within each cycle of revolution.
As an example, it has been known to provide indications related to torque, which then may be related through speed to horsepower, to provide a general indication of engine health. It is well known that horsepower is the product of speed and torque; torque, on the other hand is a function of acceleration and inertia; thus horsepower can be taken as some constant times speed and acceleration. If the acceleration and the speed are taken at the same speed, then error exists only in the fact that the resulting measured horsepower includes factors related to frictional drag and loading of the engine (such as by engine accessories). However, it is known that these factors can be accommodated by emperical formulas of various kinds.
Thus it is known to measure acceleration of an engine as an indication of its horsepower. In one technique known to the art, the engine is allowed to undergo a burst acceleration from a low speed to a high speed, the speed of the engine is monitored, and the elapsed time between sensing of first and second speeds is measured. The problem in this technique is that the speed measurement is made over an entire engine cycle, so the precise time at which the engine crosses the lower threshold speed and the upper threshold speed, thereby to accurately measure the time interval required for the engine to accelerate from one speed to another, cannot be known. In this technique, interpolation based on the difference between the average speeds sensed over succeeding cycles and the desired threshold speeds is utilized to correct the time increment otherwise measured between cycles which follow the sensing of threshold speeds. It is alleged that such a technique eliminates errors resulting from sub-cyclic speed variations which are known to occur in internal combustion engines as a result of the individual cylinder contributions to acceleration when combustion occurs and the individual contributions to cylinders to deceleration during compression for that cylinder; however, the errors resulting from gross interpolation to estimate speeds within each cycle of the engine may well exceed the cyclic variations overcome thereby.